1. Technical Field
This invention relates to an organic electroluminescent (EL) device and more particularly, to an inorganic/organic junction structure suitable for use in a device of the type wherein an electric field is applied to a thin film of an organic compound to emit light.
2. Background Art
In general, organic EL devices have a basic configuration including a glass substrate, a transparent electrode of ITO etc., a hole transporting layer of an organic amine compound, an organic light emitting layer of a material exhibiting electronic conductivity and intense light emission such as Alq3, and an electrode of a low work function metal such as MgAg, wherein the layers are stacked on the substrate in the described order.
The device configurations which have been reported thus far have one or more organic compound layers interposed between a hole injecting electrode and an electron injecting electrode. The organic compound layers are typically of two- or three-layer structure.
Included in the two-layer structure are a structure having a hole transporting layer and a light emitting layer formed between the hole injecting electrode and the electron injecting electrode and another structure having a light emitting layer and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Included in the three-layer structure is a structure having a hole transporting layer, a light emitting layer, and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Also known is a one-layer structure wherein a single layer playing all the roles is formed from a polymer or a mixed system.
FIGS. 3 and 4 illustrate typical configurations of organic EL devices.
In FIG. 3, a hole transporting layer 14 and a light emitting layer 15 of organic compounds are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11. In this configuration, the light emitting layer 15 also serves as an electron transporting layer.
In FIG. 4, a hole transporting layer 14, a light emitting layer 15, and an electron transporting layer 16 of organic compounds are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11.
Attempts have been made to improve the luminous efficiency of these organic EL devices. With the prior art device configuration, however, for reasons of poor hole injection efficiency of the hole injecting and transporting layer, it was difficult to achieve effective recombination in the light emitting layer and hence, to provide a device with a fully satisfactory efficiency.
An object of the present invention is to provide an organic EL device having an improved hole injection efficiency, improved luminous efficiency, low operating voltage, and low cost.
This and other objects are achieved by the present invention which is defined below.
(1) An organic electroluminescent device comprising
a hole injecting electrode,
an electron injecting electrode,
at least one organic layer between the electrodes, at least one layer of said organic layer having a light emitting function, and
a high resistance inorganic hole injecting layer between said hole injecting electrode and said organic layer, said high resistance inorganic hole injecting layer being capable of blocking electrons and having conduction paths for carrying holes.
(2) The organic electroluminescent device of (1) wherein said high resistance inorganic hole injecting layer has a resistivity of 1 to 1xc3x971011 xcexa9-cm.
(3) The organic electroluminescent device of (1) or (2) wherein said high resistance inorganic hole injecting layer contains at least one member selected from the group consisting of oxides, carbides, nitrides, silicides and borides of insulative metals and metalloids, and at least one of oxides of metals and metalloids.
(4) The organic electroluminescent device of any one of (1) to (3) wherein said high resistance inorganic hole injecting layer contains
silicon oxide or germanium oxide or a mixture of silicon oxide and germanium oxide as a main component, the main component being represented by the formula: (Si1-xGex)Oy wherein 0xe2x89xa6xxe2x89xa61 and 1.7xe2x89xa6yxe2x89xa62.2, and
a metal having a work function of at least 4.5 eV or an oxide thereof.
(5) The organic electroluminescent device of (4) wherein said metal is at least one member selected from the group consisting of Au, Cu, Fe, Ni, Ru, Sn, Cr, Ir, Nb, Pt W, Mo, Ta, Pd, and Co.
(6) The organic electroluminescent device of (4) or (5) wherein the content of said metal and/or metal oxide is 0.2 to 40 mol %.
(7) The organic electroluminescent device of any one of (1) to (6) wherein said high resistance inorganic hole injecting layer has a thickness of 1 to 100 nm.